This invention relates generally to a positioning apparatus, and more particularly to an apparatus for mounting a device defining a mounting plane so as to be selectively rotatable about mutually perpendicular axes with respect to a base member defining a base plane.
In order to position a device such as an antenna, mirror or flat reflector, it must be mounted with two or more axes of rotation (i.e., degrees of freedom). Accordingly, a conventional positioning apparatus has such construction as illustrated in FIG. 1, for example. FIG. 1 shows an antenna positioning apparatus using the common arrangement known as "elevation-over azimuth", also called "train and elevation", wherein a turntable 11 can rotate about an axis 1--1. This axis is vertical in a ground installation, or perpendicular to the deck if used on a ship. Two pedestals 13 and 15 on the turntable 11 support bearings which establish an elevation axis 2--2 orthogonal to the azimuth axis 1--1. (Axis 2--2 may or may not intersect axis 1--1). Antenna 17 is supported by a back-up structure 19 which rotates about axis 2--2. Antenna beam 3--3 is pointed in the desired direction by rotating the back-up structure 19 about the 2--2 axis and the turntable 11 about the 1--1 axis.
The back-up structure 19 is rotated by an electric motor and gear reduction enclosed in one of the pedestals 13. The turntable is rotated by another electric servo motor and gear train enclosed in the pedestal base 21 which is in turn secured to the ground or deck of a ship.
If the antenna 17 must be pointed with extreme accuracy, or if it must be moved with high velocity or rapid acceleration, a relatively wide servo bandwidth is required. This, in turn, demands that the natural frequencies of the structure which can be excited by the drive motor reaction torque must be high. For a given antenna moment of inertia, this implies great structural stiffness, weight, and expense.
There is a "pyramid effect" inherent in the design of any system analogous to FIG. 1. The second (elevation) axis drive requires massive pedestals 13 and 15 together with a correspondingly stiff and massive turntable 11 to obtain the required stiffness for its functioning. The first (azimth) axis drive must then not only accelerate the antenna, but it must also accelerate the elevation drive system and the structure above. Thus, in cases where high accelerations are required, the actual antenna may be only a secondary part of the load on the azimuth drive system.